Sheet buckling sheet feeding mechanisms are known wherein a normally stationary friction drive roller sits on the top sheet of a stack of sheets. When this roller is energized, it first rotates to move the top sheet rearward, forming a transverse hump or buckle in the top sheet, thus moving the sheet's forward edge out from under a hold-down blade at the front of the stack. After this buckle has been formed, and perhaps sensed by a switch, the rotational direction of the drive roller is reversed. The top sheet now moves forward, over the hold-down blade. The now separated top sheet is then fed into a sheet utilization path by downstream feed rollers, which may include a top feed roller, rotating to feed the sheet in the forward direction, and a bottom restraint roller, rotating opposite to the forward direction, to inhibit the feeding of more than one sheet.
In general such sheet feeding mechanism confines the rear edge of the sheet stack to limit rearward movement such that a buckle can be formed. This rear edge confinement may include a means which holds down the rear edge of the stack. The front edge of the stack is held down, to define a blade, clip or the like, over which the top sheet travels after the buckle has been formed, and the sheet's direction has been reversed from rearward to forward. During rearward movement, the sheet's forward edge is withdrawn from under the front edge hold-down means, as the buckle is being formed.
A common problem with known sheet feeding mechanisms is that of double sheet feeding, that is the feeding of more than the top sheet. This is due to the fact that the coefficient of friction between the paper drive element and the top surface of the top sheet varies with paper type, weight and moisture content, as does the coefficient of friction between the top sheet and second sheet in the stack. It is very difficult to adjust such a paper drive roller so that it will consistently separate only the top sheet.
The present invention provides a buckling type sheet feeding mechanism wherein single-sheet feed is reliably obtained by the use of a variable force means which operates to increase its sheet driving force as a function of feed cycle time. As a result, the top sheet begins to move rearward whenever this variable force reaches the unique required force for that particular sheet at that particular time. The force profile of this variable force means spans a force range into which many sheet types, weights and moisture contents fall. Thus, these sheets are reliably fed, one at a time, and means such as the above-mentioned restraint roller usually are unnecessary.
More specifically, the variable force means of the present invention may be provided, without limitation thereto, by a band spring whose relaxed position is out of engagement with the top sheet of a stack of sheets. To buckle the top sheet the spring is unwound such that a spring loop bows outward to engage the top sheet and move it rearward. A loop stop member is positioned to confine this loop expansion such that the rearward drive force increases as the loop size increases. When a given size loop has been formed the spring loop is allowed to quickly relax. In so relaxing, the top sheet, which has been previously buckled, is moved forward and separated from the stack.
As a further feature of the present invention, the rear of the sheet stack is held down by a second band spring. This second spring is wound in synchronism with the first-mentioned band spring to move its contact to the top sheet toward the rear edge of the sheet. In this way, the effective column length of the top sheet increases as the buckling force on the sheet increases. This effect aids in preventing a double sheet feed in almost any environment.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.